PIGMENTS 10;
or carbonizing of natural gas, oil, wood, and other organic materials. Almost none
of these products is pure carbon, but all contain mineral impurities and hydro-
carbons that are tarry in nature. Carbon makes a very stable pigment; it is un-
affected by light and air and by hot concentrated acids and alkalis; it can only be
destroyed by burning at very high temperatures. As a pigment, it has excellent
hiding power in all its forms. Oil paints made from carbon black are sometimes
slow-drying; the freer they are from tarry matter, the better they dry (see Beam,
p. 125). The organic black pigments, although they all contain carbon as their
essential constituent, vary considerably in shade and strength according to the
amount and particle size of the amorphous carbon in them.
Specifically, 'carbon black' is used to designate that produced in America by
allowing the smoky flame from natural gas to impinge against cooled, revolving
metal drums from which the black is automatically removed by scrapers (see
Cabot, p. 13, and Beam, p. 130). This product is deep brownish black in color,
strong tinctorially, and is more granular and harder than lamp black and, unlike
the latter, wets well in water.
Carthame (see Safflower).
Cassel Earth (see Van Dyke Brown).
Celite (see Diatomaceous Earth).
Cerulean Blue, which is essentially cobaltous stannate, CoO'^Sn02 (see
Church, p. 212), is made by precipitating cobaltous chloride with potassium
stannate, thoroughly washing, mixing with pure silica and calcium sulphate, and
heating. It is a stable and inert pigment and is not affected by light or by strong
chemical agents. Physically, it is finely divided and consists of homogeneous,
rounded particles which are isotropic, high in refractive index, and green-blue
by transmitted light. It has limited tinting strength, but is the only cobalt blue
pigment without violet tint. It was known at the beginning of the XIX century
as a blue compound that could be made by heating tin oxide with a cobalt solu-
tion, but not until the year 1860 was it introduced under the name, 'coeruleum/
by Messrs G. Rowney and Co., who suggested its use for aquarelle and for oil
painting (Rose, p. 289). (The word, caerukum> was used in classical times rather
loosely to indicate various blue pigments [see Bailey, I, 234].)
Chalk (whiting, lime white) is one of the many natural forms of calcium car-
bonate (CaCOs). It occurs widely distributed over the world (see Ladoo, pp.
123-130). The deposits on the English coast and those in northern France, Bel-
gium, Denmark, and other European countries are well known. It is also found
in the United States, but not in quality good enough for whiting manufacture.
Natural chalk is a soft, white, grayish white, or yellowish (iron oxide) white rock
which is largely composed of the remains of minute sea organisms (Foraminifera}.
The crude lump from the quarries is prepared by grinding with water and by
levigation to separate the coarser material. A very fine variety prepared in this
way is known as 'gilder's whiting/ Chalk is quite homogeneous microscopically;